Circular saws have long been used for sawing of wood as well as metal. It is known that in order to saw straight, saw blades with a thickness less than one percent of the diameter have to be provided with internal stresses comprising tensile stresses around the periphery and compressive stresses in the center, which is accomplished through tensioning by hammering or rolling of the central parts of the saw blade. A saw blade with correct tensioning can be made with a thickness down to 0.5% of the diameter. If in addition the thickness is varied by making the central parts thicker, the peripheral parts can be made with a thickness down to 0.2% of the diameter. Thin kerfs are economically important because of lesser consumption of raw materials and energy. Similarly, to make straight cuts with a bandsaw, compressive stresses at the edges must be avoided.
Since large circular saw blades are heavy and bulky, a need has long been felt to make them with replaceable cutting tips, to avoid removing and regrinding of the whole saw blade when one tooth is damaged. Some earlier suggested solutions are shown in U.S. Pat. Nos. 174,216, 246,703 and 303,787. This need has become stronger as more wear resistant but brittle materials have become available, such as sintered tungsten carbide, and have been used as permanently brazed tips on circular saw blades and to a lesser extent on bandsaws.
Replaceable tips have found little application on saws, restricted to thick circular saws for wood, and for thick slot milling cutters. The tip holder designs commonly used for lathe tools have not been possible to use on saw blades, for several reasons. Tip holders comprising screws require a substantial thickness to make room for a screw with sufficient strength, and the same holds for holders comprising various kinds of eccentrics.
Tip holders where the tip is elastically wedged produce compressive stresses in the vicinity of the wedged surfaces. Therefore, these wedged tips cannot be used for thin saw blades, although they have been used for turning of narrow grooves and for slot milling cutters. The use of elastically wedged tips is further prevented by the heating of the tips and their vicinity relative to the rest of the saw blade during sawing. Moreover, tungsten carbide has a lower heat expansion than steel and thus needs harder wedging than what corresponds to the cutting forces. It is known to provide slots or holes close to the periphery of the saw blade, as shown in U.S. Pat. Nos. 1,723,843 and 2,667,904, but the known arrangements have not been adequate to counteract the compressive stresses from wedging of replaceable tips.
The present invention concerns a saw blade for use with elastically wedged replaceable tips, where neither the wedging nor the heating during sawing will produce compressive stresses around the periphery, and where consequently replaceable tips can be used on this saw blade. If some teeth are damaged, the invention allows replacement of only the damaged tips, instead of removing the whole blade for regrinding or rebrazing.
A saw blade according to the present invention comprises a blade body of metal with a plurality of recesses along its periphery, where the tip is wedged between clamping surfaces, according to known art described in the published EP patent document 095 062 or the U.S. patents mentioned above. Other shapes of the clamping surfaces are also possible, and the clamping surfaces can be parallel or converge outwards or inwards. The tips can be made with or without a tapered entering portion, but should preferably have at least one convex or concave locating surface guided by a corresponding locating surface in the recess. The tips extend outside the blade body surface on at least one side.
According to the invention, the blade body is provided with gullets or slots in front of and behind each recess. The gullets or slots are substantially deeper than the recess. Radially inwards of each recess is at least one tension distributing hole with a closed contour not connected with the recess but separated from the recess by a bridge, the distributing hole is partly located closer to the periphery than the bottom of the gullets or slots.
The function of the tension distributing hole is to let the blade body parts between the gullets or slots act as two levers joined by the bridge. When the peripheral ends of the levers are pressed apart by the wedging forces during insertion of a tip, or by thermal expansion when sawing, the bridge acts as a hinge and the inner ends of the levers approach each other causing a tangential tensile stress in the blade body. This keeps the saw blade flat and increases the resonance frequency. The saw blade properties are thus improved by hard clamping or heating, in contrast to blades of the prior art which strongly deteriorate in such circumstances.
In a preferred form, the tension distributing holes are preferably shaped generally like an inverted "T", with one hole inwards of each recess, or generally L-shaped with two holes inwards of each recess.
In sawing of metal, the cutting speed is limited and there is little risk of the tips being thrown out of the recesses by centrifugal forces, but in sawing of wood with much higher cutting speed, it is suitable to secure the tips with pins or tubular pegs as shown in U.S. Pat. No. 1,608,182.
To make sure that all tips are cutting, the radial position of the tips must be well defined. This is most easily achieved by tangential supporting surfaces on the tip and on the recess. The supporting surfaces can be located at the rear of the tip as in U.S. Pat. No. 303,787, at the underside of the tip as in U.S. Pat. No. 174,216, or at the front of the tip as in U.S. Pat. No. 4,801,224. It has also been suggested to make the tip with a projection as in U.S. Pat. No. 246,703 to combine the radial positioning with securing against centrifugal forces, but that would lead to excessive deformation when removing or inserting the tips.